P0420 most times indicates an engine performance issue. Read through here: http://www.subaruoutback.org/forums/66-problems-maintenance/49537-p0420-diag.html
I had a Dodge in today that had a 420 code and it was due to a deteriorated vacuum hose on the leak detection pump for the EVAP. I heard it whistling before I even scanned the ECM for error codes.
Both problems, the idle and 420 are probably related. It could be fuel delivery, clogged fuel filter, dirty air filter, carbon build up on the IAC (idle air control motor/valve), could be dirty throttle body causing irregular throttle position readings, clogged PCV valve or misfires/poor ignition timing.
I had a Dodge in today that had a 420 code and it was due to a deteriorated vacuum hose on the leak detection pump for the EVAP. I heard it whistling before I even scanned the ECM for error codes.
Both problems, the idle and 420 are probably related. It could be fuel delivery, clogged fuel filter, dirty air filter, carbon build up on the IAC (idle air control motor/valve), could be dirty throttle body causing irregular throttle position readings, clogged PCV valve or misfires/poor ignition timing.
